The present invention relates to a process for preparing a catalyst which has a catalytically active coating consisting of high surface area, finely divided materials and catalytically active components on an inert carrier structure, by applying the catalytically active components to the finely divided materials, producing a coating dispersion from these materials and coating the carrier structure therewith.
This type of process provides catalysts which are used in many areas of chemical engineering. They are so-called supported catalysts in which the catalytically active components, in highly dispersed form, are applied to support materials in order to ensure high catalytic activity of the catalyst with the smallest possible amount of active components. For this purpose, support materials which have a large specific surface area for taking up the catalytically active components are used. They are generally finely divided, that is powdered, thermally stable metal oxides.
In the case of automotive vehicle exhaust catalysts, the support materials are applied in the form of a coating on catalytically inert carrier structures. Carrier structures which are suitable for automotive exhaust gas treatment are so-called honeycomb structures made of ceramic or metal which have parallel flow channels through which the exhaust gas can pass. In order to coat the honeycomb structure with the support materials, the support materials are generally dispersed in water and usually homogenized by means of a milling process. Milling adjusts the average particle size of the support materials to a value between 1 and 10 xcexcm.
The walls of the flow channels are coated by immersing the honeycomb structure, once or several times, in this coating dispersion, followed by drying and calcining. The final coating is also called a dispersion coating.
During this procedure the catalytically active components may be applied to the specific surface area of the support materials at different times. For example, it is known that the catalytically active components are deposited only after coating the honeycomb structure with the dispersion coating by immersing the coated honeycomb structure in an aqueous solution of soluble precursors of the catalytically active components. Alternatively, there is the possibility of applying the catalytically active components to the powdered support materials in a stage which precedes producing the dispersion coating.
The present invention relates to this second possibility of depositing the catalytically active components. In order to achieve a high catalytic activity, the type of deposition must ensure that the components are deposited in as finely divided a manner as possible on the specific surface area of the support materials. In addition the type of deposition should also lead to high thermal and ageing stability of the final catalyst, that is to say the particles of catalytically active components must be firmly fixed to the surface area of the support materials in order to prevent neighboring particles agglomerating when the catalyst is subjected to high temperatures.
A variety of processes have been disclosed for depositing catalytically active components onto powdered support materials. These include, for example, impregnation with an excess of impregnating solution. In that process, an aqueous solution of the catalytically active components is added to the powdered support material, wherein the volume of the solution may be much greater than the water absorption capacity of the support material. This results in a material with a pasty consistency which is dewatered, for example in an oven at elevated temperatures of 80-150xc2x0 C., and is then calcined at still higher temperatures to fix the catalytically active components. During the dewatering procedure, chromatographic effects may take place which could lead to uneven distribution of the catalytically active components on the support material.
During so-called pore volume impregnation, an amount of solvent is used to make up the solution of catalytically active components which corresponds to about 70-100% of the absorption capacity of the support material for this solvent. The solvent is generally water. This solution is distributed as uniformly as possible, for example by spraying over the support material while it is rotated in a vessel. After distributing the entire amount of solution over the support material the material is still free-flowing despite the presence of water. Finally the impregnated material is dried and then calcined at elevated temperatures to fix the catalytically active components on the support material. Chromatographic effects can largely be avoided when using pore volume impregnation. It generally provides better results than the process described above for impregnation with an excess of solvent.
The disadvantage of these known processes for impregnating support materials with catalytically active components is the fact that the catalytically active components have to be fixed to the support material by drying and calcining after the impregnation process, with the consumption of large amounts of energy, in order to prevent these components being desorbed from the support material during redispersion of the support material, which is required when producing the coating dispersion.
An object of the present invention is to achieve highly dispersed distribution of the catalytically active components on the support materials and largely avoid costly drying and calcining steps. Highly dispersed catalytically active components are considered to be those with crystallite sizes of less than 10 nm, preferably between 2 and 7 nm.
The above and other objects are achieved according to the invention by a process for preparing a catalyst which has a catalytically active coating consisting of high surface area, finely divided materials and catalytically active components on an inert carrier, by applying the catalytically active components to the finely divided materials, producing a coating dispersion from these-materials and coating the carrier structure therewith.
A feature of the present invention is a process that comprises the following process steps:
(a) impregnating a powder mixture of the designated finely divided materials with a solution of precursor compounds of the catalytically active components by the pore volume impregnation process, wherein the precursor compounds are adsorbed on at least one of the materials,
(b) producing an aqueous coating dispersion by using the impregnated powder mixture,
(c) coating the carrier structure with the dispersion obtained in this way and
(d) drying and calcining the coating on the inert carrier.
Materials with a high surface area within the context of this invention are understood to be those with specific surface areas (measured according to DIN 66132) of more than 10 m2/g. For treating automotive exhausts, noble metals from the platinum group of the Periodic Table of Elements are preferably used as catalytically active components. These include ruthenium, rhodium, palladium, osmium, iridium and platinum, as well as mixtures thereof.
Adsorption of the precursor compounds on the support materials depends both on the surface properties of the support materials and also on the precursor compounds and the pH of the impregnating solution. It is known, for example, that nitrates of the platinum group metals are adsorbed very strongly on aluminum oxide, but that chlorides are only weakly adsorbed at the same acidity of the impregnating solution. This difference is used during the preparation of pellet catalysts in order to have an effect on the distribution of catalytically active elements in the pellets. When using nitrates, for example, a specific outer shell profile is obtained, whereas the use of chlorides leads to almost uniform penetration of the entire pellet with the active components.
It has been shown that neither very strong nor weak adsorption leads to optimum dispersion of the catalytically active noble metals on powdered materials. In the case of very strong adsorption, the powder particles in the support materials are only inadequately penetrated by the precursor compounds. The precursor compounds are deposited only on the outer parts of the specific surface area of the powder particles. The high concentration which results in these areas leads to coarsening of the crystallite sizes of the catalytically active noble metals. In the case of weak adsorption of the precursor compounds, however, these remain mobile for a long time. When drying the impregnated support materials chromatographic effects may then take place, resulting in very uneven distribution of the catalytically active noble metals on different fractions of the impregnated material. The crystallite sizes of the noble metals then have a very wide range. In addition to very small crystallites there is also a considerable proportion of noble metals with crystallite sizes greater than 10 nm.
It has now been found that uniform and highly dispersed deposition of the noble metals is possible with a suitable combination of support materials and precursor compounds for the noble metals. This is the case, for example, when at least one of the finely divided materials has an iso-electric point between 6 and 10 and when anionic salts of the platinum group metals are used as precursor compounds. This combination of properties leads to uniform penetration of the powdered particles of the particular material and good adsorption. Adsorption in this case is substantially due to an electrostatic interaction between the positive surface charges on the support material and the negatively charged anions.
After impregnation the precursor compounds are thermally fixed on the support materials. For this, the impregnated powdered material is first dried at temperatures of up to 180xc2x0 C. and then calcined at temperatures higher than 300xc2x0 C. The precursor compounds decompose during calcination. Depending on the temperature chosen and type of precursor compound, a mixture of different oxidation states of the noble metals are formed which no longer go into solution during the subsequent process for producing the aqueous coating dispersion.
In a particularly advantageous variant of the process according to the invention, the precursor compounds are not thermally fixed. Rather, the still moist, powdered material from the impregnation process is processed directly to give an aqueous coating dispersion. By adjusting the pH of this dispersion to a value which is 1 to 3 units below the iso-electric point mentioned above, preferably between 4 and 8, the precursor compounds are prevented from going back into solution. Considerable energy savings for thermal fixing of the precursor compounds can be achieved by this method of working and the entire production process for the catalyst becomes very efficient.
To perform pore volume impregnation, the mixture of support materials chosen is uniformly rotated, for example, in a vessel while the solution of precursor compounds is sprayed over the powdered material using a nozzle. The volume of solvent used according to the invention is restricted to a maximum of 90% of the absorption capacity of the powder mixture. The small amount of solvent used prevents the precursor compounds desorbing once they have been adsorbed and thus prevents them agglomerating into larger crystallites. The smaller the volume of solvent chosen, the more reliably is unwanted desorption prevented. The volume of solvent is restricted at the lower end, however, by the requirement that the amount of precursor compounds required to load the support material to the amount desired has to dissolve in the volume which is used. This can lead to different lower limits for the volume of solvent, depending on the solubility of the precursor compounds. Solvent volumes of less than 40% cannot generally be used. Solvent volumes between 50 and 70% of the absorption capacity of the powder mixture are particularly advantageous for the process according to the invention.
If the use of a solvent volume of 90% of the water absorption capacity, due to the low solubility of the precursor compounds, does not apply the required amount of catalytically active component to the support materials in one impregnation process, then impregnation may be repeated several times with a smaller amount of solvent and appropriate intermediate drying processes.
The impregnation process must ensure, despite the low solvent volumes, that all parts of the powder mixture come into uniform contact with the impregnating solution. For this purpose, the powdered material is rotated in a vessel and the impregnating solution is sprayed over the surface of the powdered material at a constant volume flow. Volume flows of 50 ml of solution per kilogram of powdered material per minute (50 ml/(kg.min)) have proven suitable. With flows of more than 200 ml/(kg.min), the powdered material is no longer sufficiently uniformly impregnated. Below 5 ml/(kg.min), the long impregnation times are not economically viable.
The powdered material impregnated in this way is still free-flowing which greatly facilitates further processing. It is dispersed in water and optionally organic additives, either after thermal fixing or directly without further thermal treatment, in order to produce the coating dispersion for the inert carrier structures. After coating the carrier structure with the dispersion obtained in this way, the coating is dried at elevated temperatures from 80 to about 180xc2x0 C. and then calcined at temperatures of more than 300xc2x0 C.
Suitable support materials with an iso-electric point between 6 and 10 are, for example, aluminum oxide, cerium oxide, zirconium oxide, titanium oxide, silicon dioxide or mixed oxides thereof. Anionic noble metal salts which are suitable for the purpose are, for example, methylethanolamineplatinum(IV) hexahydroxide, ethanolamineplatinum(IV) hexahydroxide and hexachloroplatinic(IV) acid or mixtures thereof. Anionic salts of platinum group metals which are complexed with methanolamine or ethanolamine are particularly preferred.
To obtain the desired catalytic effect for a final catalyst it is frequently necessary that specific catalytically active components are deposited only on specific support materials, in order to avoid harmful interactions between catalytically active components and support materials. In a case like this, the different support materials in a catalyst must be impregnated separately with the relevant noble metals. Only then is a common coating dispersion produced from these materials. Thus it is known, for example, that care must be taken during the preparation of diesel catalysts, when using zeolites, that the zeolites are not coated with platinum group metals because this can lead to coking at the zeolite surface. Therefore the other support materials in the diesel catalyst have hitherto been impregnated separately with the platinum group metals before they are combined with the zeolite fractions to produce a common coating dispersion. Surprisingly, it has been shown that this type of separation is not required in the process according to the invention because the anionic noble metal salts are adsorbed to only a very small extent by zeolites.
It was found that the desired interaction between support material and noble metal salts may also be produced with support materials with an iso-electric point between 2 and 7 when cationic noble metal salts are used. In this variant of the process according to the invention thermal fixing may also take place or be omitted, as required. If thermal fixing is omitted, then the coating dispersion must be adjusted to a pH which is 1 to 5 units above the iso-electric point mentioned above, preferably between 7 and 9, in order to prevent desorption of the precursor compounds.
Various support materials and their iso-electric points are listed in Table 1, pH ranges are cited for the particular iso-electric points since the iso-electric point of a specific support oxide has different values depending on the method of preparation and thus the iso-electric points vary within a certain pH range. Thus titanium dioxide, which has been sulfatized, is more acid than pure titanium dioxide and therefore has a lower iso-electric point.
The iso-electric points in Table 1 were measured using the electrokinetic sonic amplitude process using the ESA 8000 instrument from the Matec Applied Sciences Company MA, USA. A description of the method of measurement may be found in the article by J. Winkler xe2x80x9cZeta potential of pigments and fillersxe2x80x9d in EJC, Jan. 2, 1997, pages 38-42.
Table 2 gives a few anionic and cationic platinum compounds which are suitable for the process when combined with the support materials in Table 1. The platinum complexes in Table 2 are given as examples of the analogous complexes for the other platinum group metals.
Some catalysts were prepared for use in the following examples by using the process according to the invention. For this, the following raw materials were used:
Aluminum silicate: with 5 wt. % of silicon dioxide stabilized aluminum oxide; specific surface area: 153 m2/g.
Titanium dioxide: specific surface area: 95 m2/g.
Zirconium dioxide: specific surface area: 96 m2/g.
DAY: de-aluminized Y-zeolite with a molar ratio of silicon dioxide to aluminum oxide of about 200.
Methylethanolamineplatinum(IV) hydroxide
Ethanolamineplatinum(IV) hydroxide
Platinum nitrate
Hexachloroplatinic(IV) acid: H2PtCl6 
Carrier structure: open-cell honeycomb structure of cordierite with a diameter of 2.5 cm and length of 7.6 cm; cell density: 62 cmxe2x88x922; thickness of walls between the flow channels: 0.2 mm